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Effect of Tackifiers on mechanical and dynamic properties of carbon- black-filled NR Vulcanizates.

INTRODUCTION

Tackifier is one of the most essential ingredients used in the preparation of rubber compounds for tire industries. The presence of tackifier improves building tack between layers of several compartments of a green tire namely rubber-coated fabric, breaker strip, cushion, and tread (1), Prior to vulcanization, building tack is necessary for the handing of the green tire both during the assembling and storing processes. Generally, adhesive tack can be achieved by modifying viscoelastic properties of the rubber compound (2-5). However, it is inevitable that the addition of tackifier always brings about the change in mechanical properties of the rubber.

Tackifiers used in the rubber industries can be categorized into three groups according to their chemical struc-lures (6-8), hydrocarbon resins, phenolic resins, and rosin resins. The hydrocarbon resins are commonly derived from a C5 or C9 fraction obtained as a by-product in thermal cracking of petroleum naphtha, thereby being called petroleum resins. In the case of phenolic resins, the synthesis involves the reaction between phenol or its derivatives and formaldehyde. And the rosin resins are mainly composed of a mixture of C20 such as tricyclic fused-ring namely pimaric and abietic acids.

In this work, the role of tackifier is not directly focused on its main tackifying function, but on the side effect relating to mechanical properties and dynamic characteristics, since these properties determine the performance rubber products. The dynamics properties of tackified rubbers are investigated as functions of strain amplitude and temperature. The changes in storage modulus, loss modulus, and tan delta are related to the types and loading of tackifiers. The differences in the effects of tackifiers on the dynamic characteristics measured in low-and high-strain regions of the rubbers are pointed out and explained. The obtained information is useful especially for the design of rubber compounds to achieve a particular degree of damping characteristic, since this key property partly determines the heat build-up, wet grip, and rolling resistance of tires and also controls the energy dissipation behavior of vibration absorbers and isolators.

EXPERIMENTAL

Natural rubber (STR 5L) was furnished by Union Rubber (Thailand). Tackifiers used in this research were supplied by Kijpaiboon Chemical Ltd (Thailand). The softening temperatures of employed tackifiers including petroleum resin, phenolic resin, and gum rosin were 106-108, 103-105, and 72-74[degrees]C, respectively. Ten rubber compounds were prepared by mixing natural rubber and other ingredients in the internal mixer, Brabender Plasti-corder model 350E. The formulations of individual compounds were displayed in Table 1. The mixing process was carried out by setting rotor speed at 40 revolutions per minute with starting temperature of 60[degrees]C and using filled factor of around 0.75. The sequence of adding ingredients was described in Table 2. Cure behaviors were measured by the Moving Die Rheometer (MDR). Vulcanization of the rubbers was carried out at 150[degrees]C by using hot press machine. Crosslink density was derived from Arrhenius equation after swelling the vulcanizates in toluene for 72 h. The Akron abrasion was measured as per the BS 903: Part A9 (Method B). Dynamic properties of the 2 mm-thick strips of vulcanizates were characterized in tension mode as functions of temperature and strain amplitude by the GABO EPLEXOR[R] 25N. Temperature ramp scans were tested in the range of -80 [degrees]C to 120 [degrees]C at the fixed conditions of 10 Hz and 0.1% dynamic strain superimposed on the 1% static strain. Strain sweep tests at fixed frequency of 5 Hz and three different temperatures of 20, 60, 110 [degrees]C were performed to investigate the effect of strain amplitude on the dynamic behaviors at various constant temperatures. The double-strain-amplitude sweep used in the characterization was ranged from 0.03% to 10%. The relationship between the stress response of the material to the applied strain is represented in the form of complex tensile modulus E* which can be simply expressed as [pounds sterling]* = E' + iE", where E' is the storage modulus representing elastic response of the material and E" is the loss modulus representing the viscous response. The phase angle of the complex relationship can be expressed in term of tan delta which equals to the ratio of the loss modulus to the storage one, E"/E'.
TABLE 1. Compound formulations used to study the effect of tackifiers.

Ingredient Quantity        (phr (a))

Natural rubber (STR 5L)          100
ZnO                                5
Stearic acid                       2
N220                              40
N550                              30
Aromatic oil                      12
TMQ                                2
6PPD                               2
Petroleum resin          0, 4, 8, 12
Phenolic resin           0, 4, 8, 12
Gum rosin                0, 4, 8, 12
TBBS                               1
sulfur                             2

(a.) phr, parts per hundred of rubber.

TABLE 2. The mixing sequence of rubber compounds using Brabender
Pasticorder.

Time sequence   Ingredients added
(min)

0-2nd           NR
3rd-6th         Zno + steric acid + half of carbon black + half
                of aromatic oil + TMQ + 6PPD (loaded at the same time)
7th-10th        half of carbon black--half of aromatic oil (loaded
                at the same time)
11th-12th       Tackifier
13th-14th       TBBS + Sulfur (loaded at the same time)

The mixed compound was then dumped and sheeted by using 2-roll
mill machine.


RESULTS AND DISCUSSION

The influences of type and loading of tackiliers on the vulcanization behaviors of the carbon-black-filled compounds are shown in Fig. l(a-c). The scorch times and optimum cure times of individual compounds were tabulated in Table 3. It is obvious that the presence of tackifiers in the compounds not only retarded the onset of vulcanization but also reduced achievable crosslink density as can be observed from the reduction in the difference between the maximum and minimum torque of individual cure curve. The degree of crosslink density as measured by swelling method, as displayed in Table 3, agreed well with that observation. This effect was more pronounced with increasing amount of tackifiers. An explanation to this observation could be partly attributed to the adsorption of accelerator onto the molecules of tackifiers which comprise some polar fractions (8). Thus, partial loss of effective accelerator brings about the reduction of cure state. Among three types of investigated tackifiers, gum rosin exerted most severe effect on vulcanization, while petroleum and phenolic resins exhibited comparable degree of cure interference. The observed result could be primarily attributed to the chemical structure of the gum rosin which comprises high polar and acidic portions of pimaric and abielic acids (7),
TABLE 3. Properties of untackified and tackified rubbers.

Properties        Untackified   Petro 4   Petro 8  Petro 12      Phe4

Scorch time,             4.05      4.44      4.80      5.11      4.40
[t.sub.s2],
[min]

Optimum cure             9.15      9.32     10.03     10.37      9.66
time,
[t.sub.90],
[min]

Hardness [Shore            69        69        69        68        68
A)                   [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                            1         1         1         1         1

Tensile strength         24.0      23.7      23.8      21.8      23.9
[MPa]                [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                          0.4       0.7       0.4       0.5       0.3

Elongation at             476       507       541       552       518
break [%]            [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                            9        11        14        17        12

100% Modulus              3.5       2.9       2.7       2.4       2.9
[MPa]                [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                          0.2       0.3       0.2       0.1       0.3

Akron abrasion             72        80        85        98        76
loss                 [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
[m[m.sup.3]/l               3         4         3         5         4
kc]

Crosslink                1.53      1.37      1.25      1.13      1.35
density [x           [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
[10.sup.2]               0.02      0.02      0.02      0.04      0.02
mole/[m.sup.3]]

Properties           Phe 8    Phe 12   Rosin 4   Rosin 8  Rosin 12

Scorch time,          4.92      5.10      5.15      5.41      5.87
[t.sub.s2],
[min]

Optimum cure         10.05     10.47     12.36     15.20     17.48
time,
[t.sub.90],
[min]

Hardness [Shore         68        68        68        68        67
A)                [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                         1         1         1         1         1

Tensile strength      21.7      21.9      23.8      21.7      21.2
[MPa]             [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                       0.6       0.7       0.4       0.3       0.8

Elongation at          525       534       525       553       581
break [%]         [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                         8        14        15        10        16

100% Modulus           2.7       2.4       2.8       2.4       2.0
[MPa]             [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
                       0.2       0.2       0.2       0.3       0.3

Akron abrasion          84        97        82        18       120
loss              [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
[m[m.sup.3]/l            3         5         2         5         4
kc]

Crosslink             1.24      1.12      1.29      1.14      0.97
density [x        [+ or -]  [+ or -]  [+ or -]  [+ or -]  [+ or -]
[10.sup.2]            0.03      0.02      0.01      0.02      0.02
mole/[m.sup.3]]


In general, the main function of using tackifiers is to improve the tackiness of the mixtures especially for pressure-sensitive adhesives (9-11), but the presence of tackifiers in the rubber matrix affected mechanical properties of the vulcanizates as summarized in Table 3. The hardness of tackified vulcanizates was slightly changed with the variation of added tackifier. The strength of tackified vulcanizates was progressively dropped with increasing tackifier loading as witnessed from the values of 100% modulus. When compared to that of the untackified rubber, the slight decreases in tensile strength of the tackified vulcanizates were obtained due to the compensation by the higher extensibility of the rubber network as witnessed from the values of elongation at break. The abrasion losses of vulcanized rubbers also progressively increased with the amount of added tackifiers. While the abrasion losses of the petroleum-resin- and phenolic-resin-filled vulcanizates exhibited equivalent level of deterioration, those of the gum-rosin-filled counterpart displayed much more severe effect, especially at the high loading of tackifier. The decrease in the wearing resistance of the tackified vulcanizates could be partly arisen from the lower achievable crosslink-density as mentioned before. In addition, the observed behavior could be partly come from the property of tackifier itself which are capable of increasing internal friction within the rubber matrix thereby bringing about the deterioration in abrasion loss (12), (13).

[FIGURE 1 OMITTED]

The effect of tackifiers on the dynamic characteristics of the vulcanizates was studied by analyzing the strain and temperature dependent behaviors of storage modulus ([pounds sterling]) and tan delta. Generally, the compatibility between different materials can be verified by considering its dynamic characteristics especially the variation of tan delta since this parameter represents relaxation processes of components in the mixture (14). The variation [pounds sterling]' and tan delta of the untackified and tackified vulcanizates are displayed in Fig. 2a and b, respectively. According to Fig. 2a, over the range of measured temperatures, the storage modulus of the tackified vulcanizates was higher than that of the untackified one except at the temperature above the softening point of the particular tackifier. The storage modulus of the particular tackified vulcanizate dropped significantly when the temperature passed through its softening point, 72-74[degrees]C for gum rosin, 106-108[degrees]C for petroleum resin, and 103-105[degrees]C for phenolic resin.

[FIGURE 2 OMITTED]

For the untackified case, the tan delta peak, which in general reflects the glass transition temperature (Tg) of the material, appeared at around -45[degrees]C. For the cases of 12-phr incorporation of Jackifiers, the peaks of tan delta slightly shifted toward higher temperature: -43, -41, and -40[degrees]C for the gum-rosin-, phenolic-resin-, and petroleum-resin-filled vulcanizates, respectively. This is due to the fact that the Tgs of most solid-form tackifiers lies at much higher temperature than that of the base rubber (15), (16), thereby bringing about the shift of the Tg of a particular system to higher temperature. In the molecular-level point of view, the increase in Tg implies that the presence of tackifier causes the reduction in free volume within the matrix (14), From Fig. 2b, there is only one single peak of tan delta detected for each vulcanizate, indicating the good compatibility of the particular blend.

In general, the dynamic properties of silica- or carbon-black-filled rubbers always express nonlinear behaviour as a function of strain amplitude which is well known as Payne effect (17). Similarly, the presence of tackifier molecules in the rubber matrix also results in the nonlinear behavior similar to that generated by the network of reinforced fillers. The strain-dependent characteristics of the vulcanizates filled with different loading of tackifiers at fixed temperature of 60[degrees]C are displayed in Fig. 3. It is obvious from the variation of storage modulus in Fig. 3a-c that the presence of tackifiers in the rubber matrix brought about the Payne-effect-like behavior. The nonli-nearity in amplitude domain of the untackified vulcanizates is mainly resulted from the filler-filler interaction. The breakage and recovery of the inter-aggregate network of carbon-black particles in the range of low strain are primarily responsible for the hysteresis level of the rubber. In this study, the Payne-effect-like behavior is amplified by the addition of tackifiers. The nonlinear behavior in this case is not likely to derive from the same mechanism as that happening in the case of particulate fillers since the effect can be noticed even at very low loading of tackifier at 4 phr. To explain this phenomenon, the hypothesis based on monomeric friction coefficient is applied (18, 19]. This theory proposes that within the matrix of tackilied rubber, some of the tackifier molecules situate in closed proximity to the base rubber segments thereby reducing intermolecular free volume. As a consequence, the segmental movement of the rubber molecules is more difficult thereby leading to the increase in monomeric fiction coefficient. Yuan et al. (19) displayed the calculation of monomeric friction coefficient based on the relationship to zero shear viscosity which itself theoretically associates with the entanglement and friction of polymer segments.

Although the similar behavior at low strain region could be observed by adding either carbon black or tackifier, the dynamic characteristics at high strain are completely different. While the addition of carbon black shifts the whole profile of storage modulus to the higher position, the lackified counterparts exhibit the same behavior at low strain region but reduce the elastic response at high dynamic straining. The observed behavior can be clearly seen by considering the variation of storage modulus as a function of tackifier loading. As demonstrated in Fig. 3a-c, all three types of tackifiers display the same trend of reduction in the storage modulus at high strain. This phenomenon could be attributed to the weaker network structure of the base rubber as witnessed from the measured data of crosslink density in Table 3. The decrease in crosslink density together with the increase in internal friction due to the incorporation of tackifiers lead to the higher level of damping of the vulcanizates as displayed in term of tan delta in Fig. 4a-c. It is worth noting that among three types of tackifiers, the gum-rosin-filled vulcanizates exhibit the highest level of energy dissipation.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Besides showing strong dependence on strain amplitude, the dynamic properties of the rubber largely vary with temperature especially in the case of tackifier-filled vulcanizate. This is due to the fact that the function of tackifiers changes enormously with temperature especially when the transition passes through the softening point of individual tackifier. At that point, the solid-like substrate will turn into the flowable liquid. The result of this change can be visualized by the lowering of storage modulus at high temperature. To investigate the effect of temperature, the strain sweep tests were carried out at three different temperatures, 20, 60, and 110 [degrees] C. These three temperatures were selected to cover a wide range of behaviors including the softening point of every tackifier.

From Fig. 5a-c, at temperature far below the softening points of all three tackifiers (20[degrees]C), the Payne-effect-like behaviors of the tackified vulcanizates are relatively strong as compared to the effect at higher temperatures. It is obvious that at low temperature, the degree of elastic response is related to the softening point of the tackifier. The vulcanizate filled with petroleum resin (softening point 106-108[degrees]C) exhibited the strongest Payne-effect-like behavior. And the one filled with gum rosin (softening point 72-74[degrees]C) displayed the weakest elastic response among the three types of tackified vulcanizates. At higher temperature (60[degrees]C), the storage modulus of all tackified vulcanizates dropped significantly, especially in the case of petroleum-resin-filled rubber in which the storage modulus at very low strain (<0.1%) decreased by half. The strong dependence on temperature of the tackified vulcanizates is further evidenced by observing the dynamic behavior at the temperature higher than the softening points. At 110[degrees]C, the low molecular weight tackifiers are in the flowable state having high mobility. The effect of tackifiers at this state is similar to that resulting from the presence of plasticizers in which the whole profile of storage modulus is largely shifted to the lower position. It is clearly seen that the profiles of the storage modulus of the 12-phr-tackified vulcanizates are lower than the untackified one. However, the effect of tackifier on the energy dissipation of the vulcanizate is completely different from that of the plasticizer. In general, the addition of plasticizer brings about the drop in both storage modulus and tan delta regardless of temperature (20). But, the incorporation of tackifier results in the increase in elastic response at low temperature but causes the decrease of the response at the temperature higher than its softening point. And particularly, the energy-dissipation behaviour of the tackified vulcanizates always increased regardless of temperature, as shown in Fig. 6a-c. This phenomenon discriminates the effect of tackifier from plasticizer. While plasticizer functions as a lubricant to facilitate the movement of rubber molecules, tackifier hinders that activity by increasing intermolecular friction. However, the relatively high energy-dissipation of the tackified rubbers was partly arisen from the lower achievable crosslink density. The exact conclusion about the contribution of tackifier to damping property will be verified in the future work where the degree of crosslink density of the tackified system will be adjusted to the same level as that of the untackified one.

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

CONCLUSIONS

The effects of using tackifier to improve tack property of the carbon-black-filled natural-rubber compounds have been investigated by considering mechanical and dynamic properties of the corresponding vulcanizates. Among three types of tackified vulcanizates, the gum-rosin-filled case exhibits the weakest mechanical properties when compared to those of the petroleum- and phenolic-resin-filled counterparts. The dynamic character-ization reveals the Payne-effect-like behavior of tackified vulcanizates. Although the presence of tackifiers cause the increase in storage modulus at low-strain region as do particulate fillers, the drop in elastic response at high-strain amplitude is particularly observed for the tackified vulcanizates due to the lower crosslink density. Besides, it is found that the dynamic characteristics of the tackified vulcanizates strongly vary with temperature. At the temperature above the softening point, tackifier molecules act similarly as plasticizer in term of reducing the whole profile of elastic response. However, the incorporation of tackifiers in the rubber formulations always brings about significant increase in energy dissipation behavior of the vulcanizates as witnessed from the increase in tan delta.

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Correspondence to: Woothicliai Thaijaroen; e-mail: woothici@mtec.or.th

DOI 10.1002/pen.22033

Published online in Wiley Online Library (wileyonlineIibrary.com).

[c] 2011 Society of Plastics Engineers

W. Thaijaroen

National Metal and Materials Technology Center, 114 Thailand Science Park, Klong 1, Klong-Luang, Pathumthani 12120, Thailand
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Author:Thaijaroen, W.
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9THAI
Date:Dec 1, 2011
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